The present invention relates to network interfacing, and more particularly to a novel network transceiver having circuitry for selectively referencing transmit data to a required input clock.
A Local Area Network, or (LAN), is a communication system that provides a connection among a number of independent computing stations within a small area, such as a single building or group of adjacent buildings. One type of network structure uses one or more repeaters in a star topology, with each repeater having several ports. A data packet received at one port is retransmitted to all other ports of the repeater. Each repeater in turn restores timing and amplitude degradation of data packets received at one port and retransmits the packets to all other ports.
Traditional Ethernet networks (10 BASE-T) operate at 10 Mb/s Ethernet protocol, as described by IEEE Standard 802.3; the majority of Ethernet interfaces currently operate at this data rate. However, a newer Ethernet standard, under IEEE standard 802.3 u, accomplishes the faster operation of 100 BASE-T systems, at a 100 Mb/s data rate (i.e., a 125 Mb/s encoded bit rate) using unshielded twisted pair (UTP) physical media. The 100 BASE-T standard defines operation over two pairs of category 5 UTP (100 BASE-TX) or category 3 UTP. The 100 BASE-FX network medium, covered by the 100 BASE-T standard, allows operation over dual fiber optic cabling. Ethernet protocol provides for a Media Access Control (MAC), enabling network interface devices at each network node to share accesses to the network medium. One type of connection, termed a Media Independent Interface, or MII, connects the MAC to a physical layer (PHY) transceiver configured for a particular network medium, e.g., 10 BASE-T, 100 BASE-FX, or 100 BASE-TX. The physical layer transceiver is configured for converting the MII protocol signals output by the MAC into analog network signals, such as Multiple Layer Transition-3 (MLT-3) signals for 100 Mb/s Ethernet networks, or Manchester-encoded signals for 10 Mb/s Ethernet networks.
In a conventional MII, a transmit output clock TX_CLK supplied to the repeater provides the timing reference for the transfer of transmit data TXD from the repeater to the PHY transceiver. However, the repeater may be interfaced to multiple PHY transceivers, each of which provides the repeater with the transmit clock. Since different transmit clocks TX_CLK have phase variations with respect to each other, it is difficult to provide a repeater arrangement that produces transmit data referenced to different transmit clocks.
Therefore, it would be desirable to reference transmit data TXD to an input clock shared by multiple PHY transceivers, rather than to an output clock. For example, a 25 MHz input clock used to generate internal clock in a PHY transceiver may be utilized to provide the timing reference for the transfer of transmit data. However, as discussed in our copending application Ser. No. 09/289,950 filed 4/13/1999 and entitled NETWORK TRANSCEIVER HAVING MEDIA INDEPENDENT INTERFACE OPERABLE IN A GENERAL PURPOSE SERIAL INTERFACE MODE, for 10 Mb/s data, MII can be switched into a serial interface mode, in which the PHY transceiver uses a 10 MHz clock, instead of a 25 MHz clock. Therefore, in order to support MII operations in different modes, it would be desirable to provide a network transceiver with ability to selectively reference transmit data to a required input clock.
The invention provides a novel network transceiver for transferring network signals between a repeater and a link partner in a local area network, such as one conforming to Ethernet/IEEE 802.3 Standard. The transceiver comprises a physical layer device, an interface, such as a media independent interface (MII ) conforming to IEEE Std. 802.3u, which provides the transfer of transmit data from the repeater to the physical layer device, a first clock input for providing a first reference clock signal, a second clock input for providing a second reference clock signal, and a transmit data referencing circuit that selectively employs the first or the second reference clock signal as a timing reference for transfer of the transmit data.
In accordance with one aspect of the invention, the transmit data referencing circuit is arranged to reference the transmit data to the first reference clock signal, such as a 25 MHz clock, when the interface operates in a first data rate mode of the data transfer. When the interface operates in a second data rate mode, the second reference clock signal, such as a 10 MHz clock, is used to provide the timing reference for the transfer of the transmit data.
The MII protocol requires a transmit clock output signal to be transferred from the physical layer device to the repeater. The transceiver may comprise a clock select input that enables a user to control referencing transmit data. When the clock select input is in a first state, the transmit clock output signal is employed to provide the timing reference for the transfer of the transmit data. When the clock select input is in a second state, the transmit data is selectively referenced to the first or the second reference input clock.
The physical layer device may be operable at a first and a second data rate, such as 10 Mb/s and 100 Mb/s. In accordance with a further aspect of the invention, when the physical layer device operates at the first data rate, the transmit data is selectively referenced to the first reference clock signal. When the physical layer device operates at the second data rate, the transmit data is selectively referenced to the second reference clock signal.
In accordance with a method of the present invention, the following steps are carried out for transferring network data between a repeater and a link partner:
arranging a media independent interface (MII) for transferring transmit data from the repeater to a physical layer device, which produces transmit network signals in a format appropriate for the link partner,
referencing the transmit data to a first reference clock signal when the MII operates in a first mode, and
referencing the transmit data to a second reference clock signal running at a frequency different from the frequency of the first reference clock signal when the MII operates in a second mode.